1. Field of the Invention
The present invention relates to an image obtaining method and apparatus for emitting illumination light and auxiliary light, and obtaining an image formed of reflection light of the illumination light and reflection light of the auxiliary light reflected from the observation target.
The invention also relates to an image obtaining method and apparatus for obtaining an image of an observation target irradiated with illumination light and excitation light simultaneously, which is formed of reflection light of the illumination light reflected from the observation target and fluorescence emitted from the observation target excited by the excitation light, and generating an ordinary image signal from the obtained image signal.
2. Description of the Related Art
Endoscope systems for observing tissues of body cavities have been widely known and electronic endoscope systems, in which an ordinary image is obtained by imaging an observation target in a body cavity illuminated by while light and the ordinary image is displayed on a monitor screen, have been widely put into practical use.
Recently, in the filed of electronic endoscope system using a solid state image sensor, a system that performs spectroscopic imaging by combining narrow band-pass filters based on a spectral reflectivity of a digestive organ, such as gastric mucosa, that is, an electronic endoscope system having narrow band-pass filters built therein (Narrow Band Imaging—NBI) has received attention. The system forms a spectroscopic image by employing narrow (wavelength) band-pass filters instead of a frame sequential rotating filter of R (red), G (green), and B (blue), sequentially outputting illumination light through the narrow band-pass filters, and processing the signals obtained by the illumination light beams in the same manner as in R, G, B (RGB) signal by changing the weighting on the signal. According to such a spectroscopic image, a microstructure or the like which has not been obtainable heretofore may be extracted from a digestive organ, such as stomach, large intestine, or the like.
In the mean time, unlike the frame sequential method that uses the narrow band-pass filters described above, in the simultaneous method in which microscopic mosaic color filters are arranged on the solid-state image sensor, Japanese Unexamined Patent Publication Nos. 2003-093336 and U.S. Patent Application Publication No. 20070183162 propose a method for forming a spectroscopic image by calculation based on an image signal obtained by imaging an observation target irradiated with white light. Japanese Unexamined Patent Publication No. 2003-093336 discloses a method for obtaining spectroscopic data of an observation target which do not depend on the type of illumination light, intrinsic spectroscopic property of the imaging system, and the like by obtaining estimated matrix data taken into account the spectroscopic property of the illumination light and the spectroscopic property of the entire imaging system including the color sensitivity characteristic of the image sensor, transmission factors of the color filters, and the like and performing calculation between RGB image signal obtained by the image sensor and the estimated matrix data.
Recently, in addition to the system of observing an observation target by emitting white light onto the target, the development of another type of image obtaining system has recently been underway. In this type of system, a medical agent that absorbs a predetermined wavelength, for example, ICG (indocyanine green) is administered to the observation target, then auxiliary light, which is light having a wavelength that the medical agent absorbs, is emitted to the observation target, and an image formed of reflection light of the auxiliary light reflected from the observation target is obtained, whereby a special image for observing the distribution of the medical agent in the observation target is generated.
Further, fluorescence image obtaining systems used as fluorescence endoscope systems are known, in which a fluorescence image is obtained by receiving autofluorescence emitted from an observation target irradiated with excitation light and the fluorescence image is displayed on a monitor screen together with the ordinary image described above. Such autofluorescence is emitted from an intrinsic phosphor in a living tissue. For example, if the observation target is an airway mucosa, it is thought that most of the autofluorescence is emitted from a lower layer of the mucosa, and the intrinsic phosphor can be riboflavin, tryptophan, tyrosine, NADH, NADPH, porphyrin, collagen, elastin, fibronectin, FAD, or the like.
Still further, it is known that, when excitation light in a given wavelength range is emitted onto an observation target, such as a living tissue, the light intensity/spectral shape of autofluorescence emitted from an phosphor inherent to the observation target differs between autofluorescence emitted from a normal tissue and autofluorescence emitted from a diseased tissue, as shown in FIG. 22. Fluorescence endoscope systems that make use of this phenomenon and generate a fluorescence image by emitting excitation light of a predetermined wavelength onto an observation target and detecting autofluorescence emitted from the observation target are also known. The reason why the autofluorescence emitted from the diseased tissue is attenuated more in comparison with the autofluorescence emitted from the normal tissue, as shown in FIG. 22, is presumed to be thickened mucosal epithelium of the diseased tissue, consumption of the intrinsic phosphor in the diseased tissue, or increase in the fluorescence absorbing material.
Further, as such type of fluorescence image obtaining system, for example, a fluorescence image obtaining system for generating a fluorescence image by administering a photosensitive material (ATX-S10, 5-ALA, Npe6, HAT-D01, Photofrin-2, or the like) having tumor-affinity and emits fluorescence when excited by light to a subject in advance as a luminous agent so as to be absorbed by a tumor, such as cancer, emitting excitation light with a wavelength corresponding to the excitation wavelength region of the luminous agent to the tumor, and detecting agent fluorescence emitted from the luminous agent collected in the tumor is also known.
In these fluorescence image obtaining systems, various types of comparative analysis methods have been proposed to allow an observer to accurately obtain information of tissue characteristics based on the fluorescence information. For example, when emitting excitation light onto an observation target, such as a living tissue, to obtain the light intensity of autofluorescence emitted from the observation target as a fluorescence image and displaying obtained fluorescence information based on the fluorescence image, the intensity of fluorescence emitted from a normal observation target is substantially proportional to the illuminance of excitation light, but the illuminance of excitation light decreases in inversely proportional to the square of the distance. Consequently, there may be a case in which fluorescence stronger than that of a normal tissue located remote from the light source is received from a diseased tissue located near the light source. Thus, an accurate determination of tissue characteristics of the observation target can not be made only with fluorescence intensity information.
In order to alleviate such problem, U.S. Patent Application Publication No. 20030216626 proposes a fluorescence image obtaining system for diagnosing tissue characteristics of a living body by emitting light having a wavelength range different from that of the excitation light onto an observation target as reference light, detecting the intensity of reflection light of the reference light reflected from the observation target, obtaining diagnostic information indicating a lesion site based on a fluorescence yield represented by the ratio between the fluorescence intensity and reflection light intensity of the reference light, and displaying the area of the lesion site, i.e., the diagnostic information, on the display screen of the fluorescence image in a different color, such as red.
Generally, in the image obtaining systems described above, acquisition of a special image by the emission of only auxiliary light onto an observation target or acquisition of a fluorescence image by the emission of excitation light onto the observation target and acquisition of an ordinary image by the emission of illumination light are performed in a time division manner. Then, a superimposed image is generated, for example, by superimposing the special image (or fluorescence image) on the ordinary image, and the superimposed image is displayed. When the special image (or fluorescence image) and ordinary image are obtained in the time division manner, however, the numbers of frames per unit time of the special image (or fluorescence image) and ordinary image are reduced, causing a problem that a favorable display image is not obtained when displaying the special image (or fluorescence image) as a moving picture.
Consequently, the inventor of the present invention has been engaging in the development of an image obtaining apparatus in which illumination light and auxiliary light (or excitation light) are emitted onto an observation target simultaneously, then an image formed of refection light of the illumination light and reflection light of the auxiliary light (or fluorescence) is obtained, and a special image reflecting a medical agent distribution in the observation target (or a fluorescence image reflecting a fluorescence emission intensity) from an image signal of the image.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an image obtaining method and apparatus capable of generating a favorable display image even where a special image (or a fluorescence image) is obtained by emitting illumination light and auxiliary light (or excitation light) onto an observation target simultaneously and based on reflection light of the auxiliary light (or fluorescence), and the special image (or fluorescence image) is displayed.
Further, when a fluorescence image of an observation target is observed, it is often the case that the user desires to observe an ordinary color image at the same time in order to make a comparison with the fluorescence image.
The present invention has been developed in view of the circumstances described above, and it is a further object of the present invention to provide an image obtaining method and apparatus capable of generating a quasi ordinary image signal having a low content rate of image signal attributable to fluorescence in an image obtaining method and apparatus in which illumination light and excitation light are emitted onto an observation target simultaneously to obtain an image formed of the reflection light and fluorescence.